@waleed0049: im gonna tell you how just because its basically impossible so even with the infomation i will give you you couldnt do anything with it.
hack into gmails database through some exsploit that took you years to find.
find the email of the person you want to hack.
pull their md5 encrypted password
decrypt their md5 password with a wordlist and if that doesnt work wait a year for it to be bruteforced by a hash cracker like john the ripper
log into the email and hope google doesnt stop you because youre being suspicious.